Cathode ray tubes



Feb. 2, 1960 w. E. TURK ET Al. 2,923,843

cATHoDE RAY TUBES Filed April l1, 1958 2 Sheets-Sheet 1 F/Gl . INVENTQRS'. v wam @pf 1- 7PM Feb. 2, 1960 w. E. TURK ET AL 2,923,843

CATHODE RAY TUBES Filed April 11, 1958 2 sheets-sheet 2 1N V EN TORS 3 U) TMA# faim a? uw" United States Patent() CATHODE RAY TUBES Walter Ernest Turk andl Eric Douglas Hendry, Chelms ford, England, assignors to English Electric Valve Company Limited, London, England, a company of Great Britain Application April 11, 1958, Serial No. 727,889

Claims priority, application Great Britain May 20, 1957 6 Claims. (Cl. 313-68) This invention relates to cathode ray tubes and more specifically to video signal generating tubes of the kind in which video signals are obtained by scanning with a cathode ray a target structure in which a high resistance electric charge storing plate is mounted in close juxtaposition to but spaced and insulated from a grid or mesh. Well known examples of tubes of this kind are television camera tubes such as the well known image orthicon tube.

A trouble frequently encountered with image orthicon and other tubes of the kind referred to is that of microphony caused by relative movement between the storage plate and the adjacent mesh. Such relative movement produces variation of effective capacitance with corresponding variation in picture signal output level. The movement is commonly of the mechanical resonant type and resonant vibrations are often excited by relatively small mechanical shocks orvibrations. The effect is similar to microphony in thermionic valves.

Microphony in cathode ray tubes of the kind referred to may occur in any of the tubes in practical use today, but it is obviously most apt to occur in tubes having larger diameter target structures than in others. In such tubes as at present commonly constructed, the storage plate, which is normally a thin membrane-like member of glass, is spaced from the mesh by an annular spacer or washer at the edge and between the two, the three parts-plate, spacer and mesh-being clamped together with the spacer in the middle of the sandwic An improvement as respects microphony can be achieved by replacing such'an annular spacer by a pair of strip-like spacers which are arranged as chords to the normally circular form of the target structure. Such spacers will tend to reduce microphony because they destroy the circular symmetry of the target. If, however, the usual annular washer is merely replaced by such strip spacers, there is still the practical disadvantage that during the normal de-gassing step in manufacture (in which the parts are baked in vacuo) diiferential movement between the spacers and the target frame is apt to occur and this often causes the spacers to wrinkle. These wrinkles do not disappear when, after the baking step, the tube is allowed tol cool,` and their presence of course causes the separation of the mesh and the storage plate to be larger than the desired predetermined separation, i.e. larger than the thickness of the unwrinkled, undeformed spacer. This is a defect which cannot be tolerated since, in any design of tube of the kind referred to, mesh-storage plate spacing must be as predetermined to a very close degree of accuracy. The present invention seeks to provide improved tubes of the kind referred to which shall have good anti-microphonic qualities and be relatively easily manufacturable closely to a pre-determined design.

According to this invention the target structure of a v tube of the kindV referred to comprises a high resistance charge storage plate positioned adjacent to but spaced from a mesh or grid by spacer means extending across the target structure and xed to a member which also ice serves as the support member for the grid or mesh and to which said grid or mesh is also fixed.

In one construction there are two strip-like spacers which, in the normal case of a circular target structure, are arranged as chords with respect to that structure. ln another structure there is a spacer in the shape of a rectangular frame with its corners welded or otherwise xed to the grid support member. If desired the sides of this frame may be mechanically separate, i.e. there may be four separate strip spacers welded at their ends to the grid support member, these spacers being either arranged in a rectangle or, preferably, in an incomplete rectangle leaving gaps at the corners.

Preferably the spacer means are of double construction consisting of two parts of low thermal expansion material one on top of the other and fixed together (as well as to the support for the grid or mesh) at the ends. It is found that with this double construction of spacer very good anti-microphonic qualities are indeed obtained. Where such double construction spacers are Welded to one another at their ends to form a rectangular frame it is preferred not to place one over the other to weld them together (since the result is rather far from the ideal flat spacer) but, instead, to interleave the spacers where they are Welded for, if this is done, the departure from the flat spacer ideal is less since the difference in level between the top surface of one double construction strip and that of the next one which is welded thereto, is only the thickness of one of the two parts of which each strip is composed. However, as already stated, it is preferred not to weld the ends of the strips together directly at all but to arrange them in an incomplete rectangle.

The invention is illustrated in and further explained in connection with the accompanying drawings in which: i.,

Fig. 1 is a purely schematic representation of an image orthicon tube as a Whole;

Fig. 2 is a face view of a preferred form of target structure in accordance with this invention;

Fig. 3 is a sectional elevation showing, to a considerably larger scale than is used in Fig. 2, the detailed construction of the edge of the target structure;

Fig. 4 is a View similar to Fig. 2 but with certain parts omitted for simplicity, and showing a modified arrangement of spacer strips; and

Fig. 5 is a view similar to Fig. 4 but showing a further modification of the spacer strip arrangement.

Referring to Fig. l the tube therein represented is of well known general kind and comprises within the bulbous end of an evacuated envelope E (shown broken away) a photo-cathode P on the end wall of the tube, an accelerator electrode A, a target structure T carried in the customary cylindroconical frame TF, a field mesh electrode F which serves, when the tube is in use, to ensure a uniform electro-static field, and a decelerator electrode D. The present invention is concerned with the construction of the target structure T which is purely schematically represented in Fig. 1.

Referring now to Figs. 2 and 3 the target structure comprises a tine wire mesh or grid 1, which is stretched across the aperture of a mesh support ring 2, the edge of the circle of mesh being held in a recess in the support ring 2 by means of a hat ring 3 which is welded to the face of the recess and holds the edge of the circle of mesh firmly in place. The mesh is strained liat across the aperture of the support ring 2 by being tensioned over the corner 4 of that ring `as clearly shown in Fig. 3. Parallel to and spaced from the mesh 1 is a high resistance glass charge plate 7 which is fixed at its circular edge to a so-called target sealing cranked ring 8 to which the glass is sealed. The required spacing between the mesh 1 and the target plate 7 is obtained by means of a pair of strip-like spacers arranged as chords to the target structure (see Fig. 2) and each of double construction comprising two layers of low thermal expansion material one on top of the other. The two layers composing each strip are shown at 5 and 6 in Fig. 3 and are welded at their ends to one another, the lower layer 5 (or both layers 5 and 6) being welded to a fiat annular surface provided on the support ring 2 outwardly of the clamp ring 3. The parts when assembled as shown in Fig. 3 are held together by means of a so-called backing or outer clamp ring 9 which is formed with lugs -10-four such lugs are shown in Fig. 2-which are, as the last stage in the assembly, bent out of the plane of Fig. 2 and then over the free edge of the member 8 so as to form hooklike portions as shown at in Fig. 3 and which hold the assembled parts together.

In Fig. 2, for the sake of simplicity and clarity of drawing, the mesh 1 and glass 7, which of course extend right across the central aperture of the target structure, are treated as though one could see through them both, and the references 1 and 7 are both applied to the central aperture of the target structure while the inner edge of the mesh support ring 2 and the corner 4 of that ring are both shown as circles. It will be noted that the backing ring 9 is also formed with a pair of hooks 11 (not shown in Fig. 3) which serve the usual purpose of facilitating support of the target structure in position in the tube.

The construction shown in Fig. 4 is similar to that shown in Figs. 2 and 3 with the exception that the spacer that shown in Figs. 2 and 3, dilering therefrom however,

and differing from the construction shown in Fig. 4 in that four separate strip spacers 6b lie along the sides of an imaginary rectangle, but form an incomplete rectangle, the ends of the strip spacers 6b being xed to the mesh support member 2.

We claim:

1. A cathode ray tube of the kind referred to wherein the target structure comprises a high resistance charge storage plate adjacent to but spaced from a mesh by spacer means extending across the target structure and fixed independently of said plate to a member which also serves as the support member for the mesh and to which said mesh is also fixed, said spacer means extending across said target structure inwardly of said support member and having the inner edge thereof of non-circular form.

2. A tube as claimed in claim 1 wherein the spacer means consist of two strip-like spacers.

3. A tube as claimed in claim 2 wherein the target structure is circular and the strip-like spacers are `arranged as chords with respect to the structure.

4. A tube as claimed in claim 1 wherein the spacer means comprise a spacer in the shape of a rectangular frame with its corners fixed to the mesh support member.

5. A tube as claimed in claim 1 wherein the spacer means comprise four separate strip spacers lying along the sides of an imaginary rectangle but forming an incomplete rectangle, the ends of said strip spacers being fixed to the mesh support member.

6. A tube as claimed in claim 1 wherein there are employed in the spacer means strip spacers each consisting of two parts of low thermal expansion material one on top of the other and xed together at the ends.

References Cited in the file of this patent UNITED STATES PATENTS 

